A control valve is generally used for a continuous control of a liquid or gas flow in different pipelines and processes. In a processing industry, such as pulp and paper, oil refining, petrochemical and chemical industries, different kinds of control valves installed in a plant's pipe system control material flows in the process. A material flow may contain any fluid material, such as fluids, liquors, liquids, gases and steam. The control valve is usually connected with an actuator, which moves the closing element of the valve to a desired open position between fully open and fully closed positions. The actuator may be a pneumatic cylinder-piston device, for example. The actuator, for its part, is usually controlled by a valve positioner, sometimes defined as a valve controller, which controls the position of the closing element of the control valve and thus the material flow in the process according to a control signal from the controller.
The high-performance intelligent valve positioner is an important device when a long-term control solution is required. Even though its role may sometimes be crucial, the performance of a control valve does not result solely from the smart positioner. The control valve itself must also be working well. Such problems as high friction and backlash have been associated with control valves.
Achieving loop performance starts with correct valve selection and sizing. Analyzing the installed flow characteristic is important in order to optimize control performance. Loop performance is maintained only if all the components in the control loop function correctly. It is crucial to service valves at regular intervals in order to keep the process sufficiently efficient and to maintain loop performance throughout the whole life cycle. It is also essential to predict when control valves should be serviced. Servicing valves before it is actually required could work, but it would be a rather expensive and time consuming way of doing maintenance. Waiting until valves fail and cause a possible unscheduled shutdown can also be very costly. Ideally, only those valves that really require maintenance should be serviced during a shutdown. To accomplish this, advanced valve diagnostics and/or monitoring, including online and offline diagnostics, would have to be utilized.
So-called offline diagnostics is performed during a shutdown of the process. During a shutdown, it is possible to monitor and analyze valves to check whether they need servicing or not by using, for example, a valve signature test. The valve signature plots an actuator pressure versus travel for a positioner. Comparing a recently generated valve-signature curve to the original factory curve can uncover deterioration in performance. However, this approach has problems: 1) Any view of this test shows only an instant in time; it does not provide any predictive value. 2) Because the test is performed during shutdown, i.e. the valve is not under true process conditions, the analysis result does not reflect the real situation. 3) The duration of the shutdown needs to be minimized, which means that users do not have time to test, analyze and make decisions on every valve package. 4) You cannot plan maintenance activities and the need for spare parts in advance. 5) Comparison to previous results is a tedious process.
Instead of this, it should be possible to analyze valve data just before shutdown, while the process is still in operation. In this way, those valves requiring maintenance can be pinpointed beforehand. This requires devices with online diagnostic capabilities. Online diagnostics makes it possible to monitor valve performance while the process is running, not only during shutdowns. The aim of predictive maintenance is to indicate decreasing valve performance and to warn the user before failure is so bad that it causes excessive process variability or even an unexpected shutdown. Online diagnostics can continuously monitor valve performance, but analyzing the results can be very time consuming and labor intensive. The most efficient way to carry out predictive maintenance and online diagnostics is to utilize valve controllers, which are capable of storing results in their memory and send warnings and alarms based on performance limits stored in their memory. In this way, no additional manpower is needed to analyze and study the results continuously, because the intelligent valve controller, with the help of advanced asset management software, can measure valve performance automatically. An example of an intelligent valve controller with online diagnostics is ND9000® from Metso Automation Inc.
U.S. Pat. No. 7,478,012 discloses a computerized evaluation of valve signature graphs. A control valve can be forced to make a full stroke or cycle around the entire characteristic valve signature curve during normal online operation of the control valve. However, the online full stroke test is not possible in most processes, and therefore the partial stroke tests are preferred online. The intelligent valve controller sends the signature test data to a diagnostic monitor which compares the signature data to configured boundaries.
U.S. Pat. No. 6,751,575 discloses monitoring and diagnosing process devices by collecting measured process variables and test results into a history database to be compared with process attribute information stored in databases.
In U.S. Pat. No. 5,115,672 condition or “signature” of a valve is monitored using pipe-mounted ultrasonic transducers that sense the fluid turbulence caused by the valve.